Process characteristics of semi solid forming technology for aluminum alloy casting

Semi solid forming technology is also called semi-solid processing technology. Its essence is a new material forming method to process the solid-liquid mixed metal melt containing spherulite or near spherulite solid within the solidification temperature range. Compared with conventional forming methods, semi-solid processing technology has the following characteristics

(1) The primary solid phase in the semi-solid alloy melt is nearly spherical or spherical grain, which has excellent thixotropy and rheological properties. At a very low shear rate, the alloy melt can be clamped and transported, which is similar to the solid alloy and has almost no fluidity. The viscosity of semi-solid metal melt becomes quite low under the action of high shear rate, which has the excellent flow characteristics of “shear thinning”, and can form various complex parts.

(2) As a semi-solid alloy melt being processed, it is in the phase of coexistence of solid and liquid phase. The latent heat of solidification and crystallization is released to a certain extent, which reduces the shrinkage allowance during solidification and makes it easy to carry out near net forming. At the same time, because the forming temperature is much lower than the conventional forming method, the thermal impact on the die is reduced, the service life of the die is significantly improved, the forming rate of the parts is also improved, and the production efficiency is higher.

(3) Because the viscosity of semi-solid alloy melt is higher than that of liquid metal, it is easy to control during mold filling, and it can smoothly fill into the mold cavity, without jet and turbulence phenomenon in die casting and other casting methods, and can reduce gas involvement and oxidation. The microstructure of semi-solid forming parts can be evenly distributed in the whole parts, which is less affected by the local shape and size, and has good surface quality. Its internal microstructure is dense, and there are few internal defects such as segregation, shrinkage cavity, shrinkage porosity and porosity. The mechanical properties of the parts are close to or reach the level of die forgings, with high elongation, high strength, good weldability and heat treatment Yes.

It is divided into semi-solid forming and thixoforming. Rheoforming is a process which changes the solidification state of the alloy melt by stirring or shaking, changing the heat balance state of the metal or alloy, and other methods to obtain the semi-solid microstructure during the solidification of the metal or alloy, and then directly carries out die casting or extrusion forming while maintaining its semi-solid state. This method can avoid the treatment of Semi-solid Billets, heating and slurry pouring in controllable atmosphere or vacuum, and can obtain good results for semi-solid processing of easily oxidized alloys. Thixoforming refers to the process of obtaining high-quality semi-solid alloy melt according to a certain process method, solidifying it into metal billets with spherulite or near spherulite structure, cutting the billets into the size specified by the process according to the production needs of parts, heating the prepared billets to semi-solid state, and then directly casting or squeeze casting . The main semi-solid thixoforming processes are thixoforging and thixodie casting because the billet is easy to transport and heat, and can realize automatic production. Thixotropy technology has been widely used in semi-solid industrialization, and its main defects have been reflected: the semi-solid billet mainly produced by electromagnetic stirring process has large equipment investment, high power and high energy consumption; the microstructure of semi-solid slurry is uneven and the micro segregation is serious; the secondary remelting increases the energy and material loss, and the manufacturing cost increases. Therefore, rheo forming technology with short production process and low manufacturing cost has attracted the attention of experts and scholars. Stampal spa of Italy, through NRC rheoforming equipment, mass produced automobile engine supports, and the production cost decreased by 22% compared with thixoforming process.